1. Field of the Invention
This invention relates to an optical tomograph for obtaining an optical tomogram of an object by projecting low coherence signal light beam onto the object, and more particularly to an optical tomograph which images the fine structure of the surface and a deep portion of the object on the basis of the reflected signal light.
2. Description of the Related Art
In order to obtain, for instance, an optical tomogram representing a fine structure under the eye ground retina, there has been used an optical tomograph employing low coherence light, that is, an optical tomograph which obtains an optical tomogram of an object by measuring intensity of low coherence interference light by heterodyne detection.
For example, in xe2x80x9cScience, No. 254, pp. 1178 to 1181xe2x80x9d by D. Haung et. al., there has been proposed an optical tomograph which obtains a tomogram of an object by dividing a low coherence light beam emitted from a light source which may comprise, for instance, a SLD (super luminescent diode) into a signal light beam and a reference light beam, causing the signal light beam to impinge upon an organic object while slightly shifting the frequency of the reference light beam by a piezoelectric element of the like, causing the signal light beam reflected at a depth of the object to interfere with the frequency-shifted reference light beam, and measuring intensity of the obtained interference light by heterodyne detection. In the optical tomograph, an optical tomogram at a depth at which the optical path length of the signal light beam conforms to that of the reference light beam is obtained. Accordingly, by changing the optical path length of the reference light beam by moving a movable mirror provided on the optical path of the reference light beam, the depth at which the tomogram is obtained can be changed.
However, in such an optical tomograph, the optical output of the light source is conventionally limited so that the intensity of the signal light beam does not exceed a specified value not to adversely affect the organism. Since the intensity of the signal light beam is limited, the intensity of the reflected signal light beam becomes further low. As a result, the S/N of the image data obtained becomes unsatisfactory.
In view of the foregoing observations and description, the primary object of the present invention is to provide an optical tomograph in which image data which is excellent in S/N can be obtained with the intensity of the signal light beam held at a low level sufficient to ensure safety of the object.
In accordance with the present invention, there is provided an optical tomograph which obtains a tomogram of an object by dividing a low coherence light beam into a signal light beam and a reference light beam, shifting the frequency of at least one of the signal light beam and the reference light beam so that the signal light beam and the reference light beam becomes different from each other in frequency, causing the signal light beam to impinge upon the object, causing the signal light reflected at a predetermined depth of the object to interfere with the reference light beam, and measuring the intensity of the obtained interference light, wherein the improvement comprises a light amplifier which amplifies said reflected signal light.
The expression xe2x80x9cshifting the frequency of at least one of the signal light beam and the reference light beam so that the signal light beam and the reference light beam becomes different from each other in frequencyxe2x80x9d means to shift the frequency of at least one of the signal light beam and the reference light beam so that there is generated such a frequency difference between the signal light beam and the reference light beam as to produce a beat signal, which alternately becomes strong and weak at a frequency equal to the frequency difference, when the signal and reference light beams are caused to interfere with each other.
Further, xe2x80x9cthe signal light reflected at a predetermined depth of the objectxe2x80x9d includes signal light reflected at the predetermined depth of the object and signal light reflected at the surface of the object.
The expression xe2x80x9cmeasuring the intensity of the obtained interference lightxe2x80x9d means to measure the intensity of the beat signal (the interference light) which alternately becomes strong and weak, for instance, by heterodyne detection.
Preferably the light amplifier is a light amplifier with an optical waveguide.
The light amplifier with an optical waveguide may be any light amplifier so long as it is provided with an optical waveguide. That is, the light amplifier with an optical waveguide may be, for instance, a semiconductor light amplifier, a stimulated Raman light amplifier or an optical fiber light amplifier. Among those, the optical fiber light amplifier is especially preferred.
In the optical fiber light amplifiers, those having an optical fiber added with at least one ion selected from the group consisting of transition metal ions, rare earth element ions and complex ions are preferred.
As the transition metal ion, Ti4+, Cr3+, Mn4+, Mn2+ and Fe3+ are preferred. As the rare earth element ions, Sc3+, Y3+, La3+, Ce3+, Pr3+, Nd3+, Pm3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+ and Lu3+ are preferred. As the complex ions, WO42xe2x88x92, MoO42xe2x88x92, VO43+, Pt(CN)42xe2x88x92 and WO66xe2x88x92 are preferred.
As the optical fiber light amplifiers, those having an optical fiber added with pigment may be also suitably used.
When the object is a part of an organic body, the low coherence light beam is preferred to be from 600 nm to 1700 nm inclusive in wavelength.
In the optical tomograph in accordance with the present invention, since the reflected signal light is caused to interfere with the reference light after amplified by the light amplifier, optical tomographic image data higher in S/N can be obtained without increasing the intensity of the signal light beam to be projected onto the object to such a level as can adversely affect the safety of the object. Further, by amplifying reflected signal light reflected at such a depth of an organic body that it is impossible for the conventional optical tomograph to obtain tomographic image data, it becomes possible to detect interference light of the reflected signal light and the reference light, and accordingly, the optical tomograph of the present invention can obtain an optical tomogram at a larger depth.
When the light amplifier is provided with an optical waveguide, the light amplifier can be more easily inserted into the optical path of the reflected signal light. Especially when the light amplifier is an optical fiber light amplifier, the optical fiber for amplifying the reflected signal light can be rolled and accordingly the optical fiber can be long enough to amplify the reflected signal light to a desired level without substantially enlarging the light amplifier. Further, since being very low in noise, the optical fiber light amplifier can accurately amplify weak reflected signal light.
Further, when the optical fiber is added with at least one ion selected from the group consisting of transition metal ions, rare earth element ions and complex ions, the reflected signal light can be amplified at a high amplification factor in a desired wavelength band.
Transition metal ions, Ti4+, Cr3+, Mn4+, Mn2 + and Fe3+, rare earth element ions, Sc3+, Y3+, La3+, Ce3+, Pr3+, Nd3+, Pm3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+ and Lu3+ and complex ions, WO42xe2x88x92, MoO42xe2x88x92, VO43+, Pt(CN)42xe2x88x92 and WO66xe2x88x92 are easy to add to the optical fiber and accordingly, the optical fiber light amplifier manufacturing cost, that is, the optical tomograph manufacturing cost, can be lowered by the use of these ions.
When the optical fiber is added with pigment, reflected signal light in a desired wavelength band can be efficiently amplified.
An organic body exhibits optimal transitivity and scattering properties to a low coherence light beam which is from 600 nm to 1700 nm in wavelength.